White LEDs (WLEDs) have gained significant applications in the display and general illumination market. One example is the WLED street lamp application. In another example, traditional cold cathode fluorescent lamp (“CCFL”) backlighting is being replaced by LED backlight in the LCD TV market. In such applications, as shown in FIG. 1, a large number of LEDs can be coupled in series as an LED string to provide a desired brightness. The LED string can be driven by a voltage supply as high as 200V. Multiple strings are further configured to offer the desired backlight. The serially connected LEDs have a uniform current and less power consumption than other configurations. However, if any LED in a string is damaged and becomes open circuited, the whole string is off.
A conventional solution is to bypass an open circuited LED by using a Zener diode. As shown in FIG. 1, a Zener diode triggered snapback transistor ZD is placed in parallel with one of the serially coupled LEDs A. The Zener diode ZD can have a breakdown voltage higher than a normal forward voltage of the LEDs A. Thus, in normal operation, the Zener diodes ZD are open and do not consume any power. If an LED A in the string becomes open circuited, the supply voltage VSUP (a differential voltage between Sup+ and Sup−) builds up across the open LED A, and breaks down the corresponding Zener diode ZD to conduct. Once the Zener diode ZD conducts, it triggers a snapback and clamps the voltage VA across the open LED A at a clamping voltage of the Zener diode ZD.
However, the foregoing technique has several drawbacks. First, power consumption of Zener diodes is not low. For example, the snapback clamping voltage of Zener diodes is typically around 5V and has strong dependency on manufacturing processing, operating temperatures, and conduction current levels. Also when the failed LED is returned to normal operation and/or the corresponding Zener diode ZD has a temporary false trigger (e.g., by a spike in the power supply or a current spike during LED startup), the Zener diode ZD snapbacks and cannot recover unless the entire LED string is rebooted.